The issue has the potential to pit scientists like Chyba against rocketeers like SpaceX billionaire Elon Musk, who wants to start sending settlers to Mars by the mid-2020s. When humans and all the supplies they need start arriving by the tons, there’s a risk that their biological signature could overwhelm any faint traces of ancient or modern-day life on the Red Planet.

Just in the past week, Boeing CEO Dennis Muilenburg and SpaceX President Gwynne Shotwell said they’re hopeful that humans will get to Mars within a decade. And Shotwell made clear that once that happens, the planet probably will never be the same. “It’s a fixer-upper planet,” she said at last week’s TED conference in Vancouver, B.C.

Longtime space entrepreneur Jeff Greason, who serves as chairman of the board for the Tau Zero Foundation, laid out the issue at Breakthrough Discuss in the form of a request: “If all you want to do with the solar system is look at it, the rest of us would like to borrow it for a while. … There are things to do with these bodies other than science.”

Chris McKay, a planetary scientist at NASA’s Ames Research Center, put a different spin on Greason’s request for the benefit of his scientific colleagues. “How long do we scientists try to keep the contaminators off ‘our planet’?” he asked, half-jokingly.

Carol Stoker, a colleague fo McKay’s at NASA Ames, replied that there’d have to be “a finite number of investigations — a minimum of one,” using the advanced life-detection tools that are now being developed for interplanetary missions. Such investigations should sample “a few well-chosen” sites where Martian life may lurk beneath the planet’s radiation-blasted surface.

“If it’s not life, then we have pretty good confidence that we really don’t have life on Mars, and there’s no conflict,” Stoker said. “If it is there, then we need to understand it better.”

Astrobiologists would want to learn about the biochemistry of Martian life, and whether it’s so different from earthly life that it represents a truly alien genesis rather than a case of cross-planetary microbe-swapping. They’d also want to see how it interacts with organisms from Earth, and whether such interactions have negative effects.

Finding out what one form of life does to the other would be crucial for planetary protection, not only for Martian organisms but for humans and other Earth-based life as well. No one wants to see the kind of scenario depicted in movies ranging from “The Andromeda Strain” to “Life.”

On one side of the debate, some would say the risk of doing harm Martian life is so great that human habitation should never be allowed on the Red Planet. They may point to the 51-year-old Outer Space Treaty, which obligates the U.S. and other signers to avoid the “harmful contamination” of other celestial bodies.

On the other side, proponents of Mars settlement say that the niches where Martian life could exist are most likely deep underground and so isolated that human activities would pose little risk. “You could terraform Mars, and the microbes on Mars would survive,” said Robert Zubrin, founder and president of the nonprofit Mars Society.

Some argue that earthly life has already arrived on Mars, via meteorites as well as NASA’s landers and rovers. There’s even a hypothesis stating that earthly life got its start from space-borne Martian microbes traveling the other way. Therefore, it’s too late to worry about another biological invasion, they say.

Princeton’s Chyba advocates for a middle ground, which he calls the Smokey the Bear argument: “Until we know more, let’s be careful.”

“Once we start sending humans to Mars, we shouldn’t just concede that, ‘Well, OK, we’re going to ruin the biosphere now,’” he told GeekWire. “First of all, there ought to be steps that we can take to minimize our impact.— and that will have to do both with where we land and also should have to do with technology. We should take the view that we should try to minimize the number of microbes that we’re expelling from our structures and that are leaking out of our spacesuits.”

If scientists rule out the existence of indigenous Martian life, then the safeguards might not need to be as airtight, Chyba said.

The good news for astrobiologists is that the clock may not be ticking down as quickly as Shotwell and Muilenburg make it sound. Timelines for Mars missions are notorious for slipping to the right, as evidenced by the case of the Mars sample return missions that were supposed to have flown more than a decade ago. The first crewed trip to the Red Planet may well be delayed until the 2030s, which is more in line with NASA’s stated schedule for Mars exploration.

Who knows? By the time the first Mars-bound astronauts step inside SpaceX’s BFR rocket ship, NASA’s Orion deep-space capsule or some other spacecraft, there’ll have been multiple robotic missions to resolve the life-on-Mars question once and for all — and enough time for space officials, scientists and settlers to agree on the rules of the road.